Study: 30 Miles/Week May be 'Excessive' After a Heart Attack

In a statistical analysis of runners who have had a heart attack, those running more than about 30 miles a week have a much higher mortality rate than those running less.

The new issue of the medical journal Mayo Clinic Proceedings, published today, brings together for the first time two of running's most respected researchers, Paul Williams, Ph.D., and Paul Thompson, M.D., and a trio of cardiologists led by James O'Keefe, M.D., who have written extensively about the "excessive exercise" hypothesis in recent years. The 30-mile limit reported in Mayo Clinic Proceedings coincides closely with what O'Keefe has been suggesting.

“[This is] the first data in humans demonstrating a statistically significant increase in cardiovascular risk with the highest levels of exercise,” write Williams and Thompson. "Our analysis suggests that the benefits of running and walking do not accrue indefinitely, and that above some level, perhaps 30 miles a week of running, there is a significant increase in cardiovascular mortality."

Thompson, however, added an important caveat when interviewed by Runner's World Newswire.

“It's a big deal that these were cardiac runners," he said. "We can't say that the same results will apply to healthy runners. The two groups are very different."

In an accompanying editorial, cardiologists O’Keefe, Chip Lavie, and Barry Franklin note that the Williams-Thompson data mirror that of a recent German study of patients with stable coronary heart disease. Both studies reached the same key conclusions: that “physically inactive individuals were at the highest risk for adverse health outcomes; and the most physically active group was at an increased risk for cardiovascular mortality compared with moderately-active individuals.

In statistical parlance, both studies uncovered a "reverse-J-shaped curve” for deaths. That is, some exercise was better than none, but too much exercise appeared to raise risks again, though not to the level of the non-exercisers.

In an email to Newswire, O’Keefe wrote: “The evolving picture on exercise is becoming more clear: no other lifestyle factor confers more benefits to health and longevity than does regular moderate physical activity, but excessive exercise may erode some of the benefits.”

The Williams-Thompson study followed 2,377 runners and walkers who had suffered a prior heart attack. After a period of 10.4 years, mortality rates from cardiovascular disease among these subjects fell steadily as weekly running mileage increased: by 40 percent for 18 miles/week runners, and by 60 percent for 27 miles/week runners. "These risk reductions were significantly greater than the 21 percent risk reduction achieved by merely satisfying current exercise recommendations," they write.

However, when the runners being studied ran more than 30 miles/week, their mortality risk jumped by a factor of 2.6 compared to those running in the mid-20s/week. At 30+ miles/week, their risk was roughly equivalent to those running the recommended 7 to 9 miles a week. Only about six percent of the subjects attained this 30+ miles/week level. (The researchers used MET hours/day to measure subjects' exercise. It placed the sharp rise in risk at 7.2 MET hours/day, equivalent to 4.5 miles a day, or 31.5 miles per week.)

Williams combines walkers and runners, because he has previously shown that they have similar health profiles when they burn equivalent calories in their weekly exercise routines. He has been conducting running and walking research since the early 1990s with his National Runners & Walkers Health Studies. Thompson has finished more than 25 Boston Marathons, and has been researching and writing about exercise and heart health since the mid-1970s.

Williams told Newswire that he first noticed the increased risk for higher-mileage runners several years ago. “I thought, ‘Oh, crap,’ there must be a problem in my data entry,” he said. He believed this because he had previously reported that running 40 to 50 miles a week appeared to diminish heart disease and risks of heart disease.

When Williams double-checked his new data, he found no errors. He says he doesn’t know what is causing the jump in mortalities, but believes it’s something different from traditional cardiovascular risk factors such as cholesterol and high blood pressure. He also notes that he is seeing similar trends among healthy runners, but there are so few deaths that it is difficult to draw statistical conclusions.

The Williams data does contain a curious anomaly. The runners reaching 30 miles a week were much younger (average age 51) than those doing fewer miles. Runners logging around 20 miles/week were 59, and those under 10 miles/week were 67. On the one hand, this makes sense, because most runners run less as they age. On the other hand, younger runners should have fewer heart attacks than older runners.

This raises the specter of “reverse causality.” The higher-mileage runners apparently had a heart attack at age 38. (All groups reporting having their heart attack 12 to 14 years prior to joining the study.) This is young for a first heart attack, and implies a serious disease state. Is it possible that these runners began running more in the hope that it would prolong their lives? If so, then it’s also possible that they died from their aggressive heart disease, not from their running mileage.

“We can’t know details about the severity of the runners’ cardiac disease, which are very important predictors of survival,” researcher Chip Lavie wrote in an email to Newswire. “We did see that the younger runners were using less lipid and blood-pressure medications, which could have been protecting the older subjects.”

O’Keefe, Lavie and colleagues are now using the term “cardiac overuse injury” to describe heart problems related to possible overexercise, clearly borrowing from the musculoskeletal injury domain familiar to most runners. In their editorial, they claim that “approximately one of 20 people is overdoing exercise.”

That claim is a stretch. According to a well-known federal study called NHANES (National Health And Nutrition Examination Survey), only 3.5 percent of American adults achieved the recommended amount of exercise in 2004 (equivalent to about 7 miles/week of running) when they wore accelerometers to give an objective measurement of their activity. This means that the percent of Americans running more than 30 miles a week is almost certainly well under one percent.

If the excessive exercise picture is becoming more clear, which is not universally accepted, it begs an explanation. So far the mechanism has proven elusive. Why does “excessive exercise” lead to increased heart issues? O’Keefe and others have pointed to possible heart scarring from prolonged, intense exercise, such as a marathon run. This has led some to coin the term “Pheidippides syndrome.” Heart scars could set off rhythm disturbances such as atrial fibrillation or more dangerous rhythm malfunctions.

However, Williams has previously reported low rates of atrial fibrillation among runners in his very large database, and no increase with increased mileage, intensity, or marathon participation.

Nonetheless, all the cardiologists Newswire contacted about the new Williams-Thompson study said that they see many endurance athletes with atrial fibrillation in their daily practices. Most of these athletes are in their 50s and 60s. Thompson added that he suspects few death certificates “are coded for rhythm disturbances.” The high-mileage runners in the new study did run more marathons--an average of three every four years--than the modest-mileage runners.

Importantly, studies such as the National Runners and Walkers Health Studies can’t tell what would have happened if the 30+ miles/week runners had exercised differently. What if they had run only 10 miles/week? What if they had run 50 miles/week? The data can’t answer these questions.

Newswire contacted several statisticians with a running background to ask their opinion about the sharp jump at 30+ miles in the new paper.

"The authors divided the participants into six categories, with predetermined boundaries such that the highest category, those exercising more than 7.2 MET hours a day, includes a small proportion of the participants, and a very wide range of individual variation," replied University of North Carolina professor Richard Smith, Ph.D., an 11-time Boston Marathon finisher and director of the Statistical and Applied Mathematical Sciences Institute. "I feel it would have been better if they had used the individual exercise levels, and then generated the best curve to fit them. There's nothing special about the 7.2 cut-off point."

“There may be something going on with the highest exercising group, but it's not clear what,” adds Bill Billing, a frequent Boston Marathon runner and retired clinical trials statistician at Pfizer. “The apparent suddenness in the loss of benefit at 7.2 MET hours/day makes no sense. It's an artifact of the statistical models and the manner of presentation."

A third article in the Mayo Clinic Proceedings investigated the general question of: How long do elite athletes live? The researchers concluded, “The evidence available indicates that top-level athletes live longer than the general population.” The paper may have little relevance to endurance athletes, however, as 85 percent of the subjects were members of professional football, baseball, and soccer teams."

Initial data from a large exercise and mortality study were presented several months ago in abstract form at the annual meeting of the American College of Sports Medicine. There, the National Cancer Institute’s Hannah Arem, Ph.D., pooled data from six large, prospective studies. She ended up with 651,001 subjects who were followed for an average of 11.9 years, during which time 116,692 died. Her results showed no additional mortality benefit among those exercising beyond 3.2 MET hours/day (about 14 miles a week of running) but also no increase in risk up to 10.7 MET hours/day, about 47 miles a week of running.

In other words, she observed no reverse-J-shaped curve trend among the heaviest exercisers.

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